Digital electronic systems based on, for example but not limitation, microcontrollers, microprocessors, digital signal processors, programmable logic arrays (PLA), hardwired digital logic and the like, often use a non-volatile memory for permanent storage of program and control data. A preferred type of non-volatile memory is an electrically erasable programmable read only memory (EEPROM). The process of programming an EEPROM cell is lengthy in time compared to the time required to program data to volatile memory such as static and dynamic random access memory (SRAM and DRAM, respectively). In addition, to prevent corruption of programmed data in the EEPROM it is necessary to maintain a minimum programming voltage level throughout the programming cycle. If the programming voltage level is too low, insufficient charge is transferred to the EEPROM cell, causing a "soft programming" of the EEPROM cell. A softly programmed EEPROM cell will loose its charge over time and thus the correct information stored in that memory cell will be corrupted.
In the EEPROM structure, a pair of polysilicon gates are separated by a silicon dioxide layer. In the operation of the EEPROM, a voltage of suitable magnitude is applied across a very thin gate oxide layer which induces tunneling of electrons between the semiconductor substrate and the lower gate of the EEPROM memory cell. The transfer of electrons alters the threshold voltage and consequently the conductivity of the cell, thus determining the logical state of the data stored in the cell. It is imperative that the power supply voltage of the EEPROM be maintained at a sufficiently high voltage level to assure that the programming voltage level is adequate for programming the EEPROM cell completely. Maintaining a sufficient voltage level is especially important in low voltage applications such as battery operated systems using complementary metal-oxide-silicon (CMOS) devices designed to operate at voltages of less than 6 volts and even below 2 volts.
Corruption of data in the EEPROM is of particular concern in applications where the information is critical and the power supply is not always reliable, such as in a battery supply where the battery voltage is subject to low voltage conditions due to discharge of the battery. One such application is in security systems where long term memory without corruption is imperative. Security systems may include vehicle, home and office systems where a low battery condition could occur just when critical data is being programmed to the EEPROM. A security system, especially with a battery supply in a vehicle, may rely on security codes or status flags to determine whether a user is authorized and it is very important that the stored information remain uncorrupted and valid.
Therefore, what is needed is a system, method and apparatus for determining that a programming voltage level is sufficient for reliably programming an EEPROM having non-volatile storage data.